Within a telecommunication switching system, telecommunication switching systems use signals to communicate control information between themselves. It is necessary that the circuit receiving the signals have signal-recognition facilities (referred to as call classifiers) for detecting and recognizing various signals. Conventionally, many of these signals have been cadence signals: a pattern of signal energy on and signal energy off periods, wherein the pattern may be repeating or non-repeating. Two techniques are in call classifiers. The first technique utilizes a state machine for the signal recognition. The state machine has typically been very complex, involving large number of states even for signaling schemes that use simple cadences, and has to have been individually designed for recognizing signals used in each particular telecommunication system or country. Such a state machine is difficult and expensive to design, implement, modify or adapt for different signaling schemes. In addition, the presence of noise in the signals that are to be recognized causes the state machines to produce erroneous results. This noise problem has been solved in the prior art using classical filtering techniques employed ahead of the state machine signaling detection stages. The time variant characteristics of the noise often make such attempts at this type of filtering impractical.
The second technique that has been utilized to detect call classifiers is through the use of cadence tables. This technique uses an incoming signals cadence—its intervals of silence and energy, as an index into tables of candidate values. The table can contain a myriad of values whose outcome could lead to the detection of several different signals. In the presence of noise, the cadence tables yield many misses that would be interpreted as the identification of an unknown signal and the consequent disconnection of the classifier. These missed-identifications reduce the accuracy of tone detection.